A known hydrometallurgical process for the production of copper cathodes from their oxide ores typically involves heap leaching, solvent extraction and electrowinning. A known process for copper cathode production from sulphide ores typically involves concentration or flotation of the ore, a pyrometallurgical process such as roasting or smelting, conversion then electrorefining.
Known electrowinning and electrorefining process which produce copper cathodes typically plate copper to a stainless steel substrate or copper starter sheet for several days, (eg. up to 7). Typically, current densities of between 250 and 320 A/m2 are used. Anodes used in electrowinning may be lead alloy based.
A problem in electrowinning and electrorefining processes is dendrite formation, or formation of surface nodes or irregularities, on the copper cathode surface, that ultimately reduces the copper cathode production and copper quality/purity. The formation of dendrites closes the gaps between the cathodes and anodes within a cell and ultimately leads to short-circuiting if the dendrite reaches the anode, therefore the production of copper cathodes is decreased. If the dendrite reaches the anode it can also cause lead sulphate to flake off the anode and contaminate the cathode and/or the electrolyte solution. Moreover, the uncontrolled growth of dendrites precludes increasing the current density used in the tankhouse since the increased dendrite formation at higher current densities will further reduce the plant current efficiency. Uncontrolled dendrite formation, porosity and voids are also believed to reduce the purity of the copper cathode. It is thought that the electrolyte, which contains small amounts of iron in solution, lead in solid form and other impurities, is trapped in the inclusions formed during dendrite formation, and the more uncontrolled the dendrite formation the higher the level of impurities in the cathode.
Dendrite formation, porosity and/or voids of the copper deposit are directly related to nucleation and growth which may be controlled by the addition of organic additives. Known current attempts to optimise the electrodeposition of copper typically involve the addition or dosing of organic additives and chloride ions into the electrolyte. Animal glue (a levelling agent) and thiourea (a grain refiner) are commonly added to the electrolyte in electrorefining cells. Levelling agents eliminate surface roughness by inducing preferential metal deposition in surface recesses, resulting in the removal of surface height irregularities. Grain refiners induce new nucleates on the surface recesses of the electrode from which new crystals may start to grow. Chloride ion additions are known as a depolarizer. Chloride ions are understood to enhance the rate of charge-transfer, growth rate, or the formation of dendrites rather than the nucleation rate.
An organic additive added during electrowinning is guar gum, which is believed to act as a weak levelling agent and a brightener. However, formation of dendrites is not effectively controlled through the addition of guar gum and chloride ions to the electrolyte.
U.S. Pat. No. 5,733,429 describes the use of polyacrylic acid (repeating polymeric units having the structural formula of —(CH2—CH(COOX)—CH2)n— wherein X═H, periodic table group 1 or group 2 element salt, an ammonium salt or mixture thereof) to an ionic solution of copper for electrowinning to form a copper cathode. U.S. Pat. No. 5,733,429 discusses the formation of dendrites or surface nodes on the cathodes and the increased potential to short-circuit the cell. U.S. Pat. No. 5,733,429 also teaches the use of polyacrylic acids, alone, to prevent the formation of dendrites, to minimize anode flaking, and to prevent shorts in the circuit.
In U.S. Pat. No. 5,733,429 the preferred concentration of polyacrylic acid in the electrolyte is between 10 and 200 mg/L. Such a high level of dissolved organic material in electrolyte is known to cause significant carbon and hydrogen contamination of the copper cathode. This increased contamination is known to cause frequent breakages during the drawing process of the copper wire (see, for example, Chia, E. H., et al., Copper Rod and Cathode Quality as Affected by Hydrogen and Organic Additives, Wire Journal International, November 1992). Furthermore the process of U.S. Pat. No. 5,733,429 operates at current densities of 130-204 Amp/m2 (12 and 18.75 Amp/ft2) for electrowinning and 218 Amp/m2 (20 Amp/ft2) for electrorefining. These are very low current densities and, as such, do not require the addition of guar gum or glue. Thus U.S. Pat. No. 5,733,429 does not provide a solution to prevent dendrite formation in electrowinning or electrorefining processes involving high current densities.
U.S. Pat. No. 6,183,622 describes the use of a tertiary alkylamine of polyepichlorohydrin to improve the ductility of copper which is electrowon, electrorefined or electroplated. U.S. Pat. No. 6,183,622 does not teach that such additives have any effect on dendrite formation, but does suggest that the addition of “organics” to the electrolyte solution may interfere with the deposition process and that care needs to be taken in selecting additives to an electrolytic cell.
U.S. Pat. No. 6,284,121 describes the addition of a range of additives having molecular weights in the range of 200,000-10,000,000, in particular poly(sodium styrene 4 sulfonate), to an electroplating system for electroplating copper onto microelectronic components. The additive is described as preferentially adsorbing onto protruding surfaces such that deposition occurs without voids. The overall effect of the additive is to assist in the production of a smooth surface on the electrodeposited copper being plated onto the circuitry. U.S. Pat. No. 6,284,121 does not teach or predict that addition of high molecular weight compounds to the electrolyte solution prevent the formation of dendrites when electrowinning copper cathodes.
U.S. Pat. No. 2,798,040 describes the electrowinning of zinc and copper in the presence of acrylamide polymer (homopolymer and copolymers of acrylamide) by dissolving the acrylamide polymer in water or electrolyte, or adding in a solid form, to the copper electrolyte at a concentration of 25 to 150 mg/L to accomplish an improved deposition of metal. The electrolyte is described as containing 20-70 grams/L copper with a substantial proportion of sulphuric acid, and is essentially free of chloride ions, to obtain smooth bright copper deposits after 5, 13 and 16 hours of electrowinning at 25° C. and 172 Amp/m2 (16 Amp/ft2) current density.
Vereecken and Winand (Vereecken J. and Winand R., Influence of Polyacrylamides on the Quality of Copper Deposits from Acidic Copper Sulphate Solutions, Surface Technology 1976; 4:227-235) compared the influence of nonionic and cationic polyacrylamides (PAM) and Guar Gum (Guar) on the quality of copper deposits using “industrial” copper sulphate solution at 200 Amp/m2 and 50° C. The electrolyte composition disclosed was, in g/L: copper, 50; Mn, 10; Mg, 4; Co, 1.5; phosphate ions, 10 and sulphuric acid, 50. Moreover, it is unclear in the paper whether PAM was prepared in water or at pH 3. Every 12 hours, 1 mg/L of PAM was dosed for 48 hours of electrowinning. This study concluded that the quality of the copper deposits obtained using Guar was always better than those obtained with both nonionic and cationic polyacrylamides and that the polarization behaviour (that is the variation in cell voltage at a constant current of the electrode) remains constant irrespective of the presence or absence of both nonionic and cationic polyacrylamides.
Vereecken and Winand conclude that, “Addition of polyacrylamides to pure acidic copper sulfate baths improves the quality of the deposit. In particular, with cationic polyacrylamides (10-20% cationic conversion) the deposits are smoother and contain thin elongated crystals oriented in the electric field. The viscosity of the 0.5% polyacrylamide aqueous solution at pH 3 also seems to play an important role: we found an optimum about 4 cP. On the other hand, the presence of polyacrylamides does not change the cathodic galvanic potential, the current efficiency and the orientation of the texture of the deposit. These inhibitors are not incorporated in the deposit. However, it should be noted that for all our experiments the quality of the deposits obtained with polyacrylamides was never as good as with guar gum.”
Whilst both U.S. Pat. No. 2,798,040 and Vereecken and Winand (1976) describe the use of polyacrylamide in electrowinning, neither discloses the use of an electrolyte with chloride ions being present. Furthermore, Vereecken and Winand use a sulphuric acid concentration of 50 g/L, which is below typical plant operating parameters. It is highly unlikely that the processes of U.S. Pat. No. 2,798,040 and Vereecken and Winand could be applied to a commercial plant electrowinning process, since currently used organic extractants, e.g., ACORGA® M5640 or LIX® 984 require sulphuric acid concentrations from 150-200 g/L to strip the copper ions back from the organic phase to the aqueous phase. This aqueous solution comprises the advance electrolyte and is mixed with a recirculating electrolyte to then be fed to the tankhouse. The electrolyte in typical commercial copper electrowinning cells throughout the world contain 150-200 g/L sulphuric acid.